BEWARE: Sadly, Belsize Park is targeted by thieves on bikes snatching people's phones so please be very careful, especially if you're visiting at night.
At the coordinates, you will see a white circular turret. Belsize Park is not the only place where this structure can be found – you can spot the same type of building e.g. in Chenies Street near Goodge Street station. They are entrances of deep-level shelters built between 1940 and 1942.
Ten shelters were originally planned but the one at St. Paul's station was never built due to concerns about the stability of the buildings above while works on the one at Oval station stopped due to difficult ground conditions. The completed ones are at Chancery Lane station on the Central Line and Belsize Park, Camden Town, Goodge Street, Stockwell, Clapham North, Clapham Common and Clapham South on the Northen Line.
Plan of Belsize Park deep-level shelter
Image source: Subterranea Britannica
The shelters were built with the option to take over for railway use after the war. Their positions were chosen on routes of possible north-south and east-west express tube railways and it was decided that each shelter would consist of two parallel tubes. Like most of the shelters, Belsize Park has two circular turrets on the surface that give access to a lift and spiral staircase leading down to the twin tunnels. The given coordinates take you to the southern entrance. Nowadays it is used for storage as you can witness it at the GZ.
Henry Moore: Shelterers in the Tube (1941)
Image source: Wikimedia Commons
Many Londoners sought shelter in the tube stations themselves. Such scenes became the theme of the drawings of Henry Moore who lived in Belsize Park at the time. Moore usually went to town by car but one September night in 1940 he and his wife came home using the tube where they saw people lying on the platforms at all stations and when they got to Belsize Park they were not allowed to leave the station for an hour because of the bombing. This experience moved and inspired Moore to visit tube stations and create drawings of the scenes he witnessed there.
Sources & further reading:
BARR, R. (Comp.). Belsize Remembered: Memories of Belsize Park. London : Aulis Publishers, 2018.
Subterranea Britannica: Belsize Park Deep Level Air Raid Shelter. URL: http://www.subbrit.org.uk/sb-sites/sites/b/belsize_park_deep_shelter/index.shtml [Accessed on 12/02/2019].
Wikipedia: London Deep-Level Shelters. URL: https://en.wikipedia.org/wiki/London_deep-level_shelters [Accessed on 12/02/2019].